1. Field of the Invention
This invention relates to improvements in radio altimeters, and, more particularly, to a method and apparatus for automatically adjusting the resolution of a digital radio altimeter over its range of altitudes of operation.
2. Description of the Prior Art
Radio altimeters of the CWFM type have been known for a number of years. Until recently, the data processing techniques which have been employed in conjunction with such altimeters have been for the most part analog. Recently, a digital CWFM radio altimeter processor was proposed in U.S. patent application Ser. No. 800,684, contemporaneously filed herewith, entitled "Radio Altimeter and Method for Operating Same" and assigned to the assignee hereof.
Briefly, such digital CWFM radio altimeters transmit a signal frequency modulated with a shaped waveform and detect the reflections from the underlying terrain. The reflections, delayed by a time proportional to the two-way travel time of the signal, are detected, then mixed with the currently transmitted signal to produce a signal of frequency equal to the difference between the detected reflections and the transmitted signal. The mixed signal is then digitized, and its period determined. The period of the mixed signal is then utilized to directly determine the altitude of the aircraft with which the system is used.
In determining the period of the mixed return signal, it is compared with a calibration frequency derived, for example, from an oscillator operating at a known frequency. Thus, the number of oscillator periods occurring within a period of the mixed return signal is determined for indicating the height of the aircraft. At lower altitudes, the period of the mixed return signal is large, thereby encompassing a large number of clock periods, enabling accurate low altitude measurements to be made. At higher altitudes, on the other hand, the period of the mixed return signal is relatively small, encompassing few, if any, of the clock periods. Consequently, in order to accurately resolve higher altitudes, it would be necessary to use a comparison clock frequency which is relatively high. High frequency clocks, however, are incompatible with presently available MOS devices, and must be implemented with high speed TTL devices for an economic realization of the system. That is, to effect reasonable resolution at altitudes over which radio altimeters are ordinarily used, for example, to 2500 feet, clock frequencies on the order of 25 MHz are necessary. Such frequencies are beyond the range of MOS device technology, without undue expense.